This invention relates to a drill bit used in percussion drilling and particularly percussion drilling in which a tool capable of exerting rapid percussive forces is mounted directly above the drill bit and is operated by a compressible fluid such as air or some other gas. The drill bit consists of an anvil portion and a head portion. As the percussive tool strikes the anvil on the bit, the assembly is rotated so that the cutting inserts, typically of a hard metal, such as tungsten carbide, are indexed to a new part of the formation being drilled upon each blow. The high frequency of percussive blows on the formation being drilled causes the formation to fail. The net penetration of each insert into the formation per blow is, however, very small and therefore the interrelationship between one insert and another becomes a critical factor.
Gage wear on percussion bits is a critical problem in that the inserts cutting on gage, that is the outer diameter of the hole being drilled, are required to remove the maximum material volume as well as being subjected to sliding abrasive wear as the bit indexes. The provision of a greater number of inserts on gage generally requires a reduction in the volume of support material retaining each insert or a reduction in the areas of the fluid (in particular, air) flow passages provided for cutting debris removal from the inner rows of inserts on the bit face. Reduction of the fluid flow passage area has the detrimental effect of increasing the back pressure on the percussion tool so that the impact force per blow is reduced, hence the penetration rate decreases. Increasing the number of inserts in the gage row while maintaining the fluid flow passageway area reduces the support material per insert so that fatigue failure of the support material generally results.
A typical known percussion bit is characterized by a high wear area and a low wear area because the anvil portion of the bit, being enclosed within the percussion tool casing, generally is not worn out or damaged at the same rate as the drilling head portion of the bit. Provision of a separate cutting and anvil portion with a threaded means of interconnection has not generally proven to be effective due to fatigue failure of the threads. The space available limits the physical thread size as well as the variations in retention methods that can be utilized while allowing `in-field` separation of the head portion of the bit from the anvil portion.